Equalization of carrier transmissions



.Hume 24 1924.

3,498,568 H. s. osa-.ORNE

EQUALIZATION OF CARRIER TRANSMISSIONS Filed July 9. 1920 Zowlaas 20 22 II mVENToR. /fei\\ Y M IV A TTORNEY 1 number of different causes.

Patented June 24, 1.924.

um'rsu STATES] PATENT orrlcs.

nanou: s. osBoaNE, or NEW Yonx, N. Y., AssmNon To Aumont Turmalin AND'rnLEeiuuHl COMPANY, A coomrnon or NEW You.

l y, EoUALrzA'rIoN or cAnnInn 'rnANsmssIoNs Application led July 9,1920.

To all whom it may concern."

Be it known that I, HAROLD S. OsBoRNE, residing at New York,'in thecounty of New York and State of New York, have invented certainImprovements in VEqualization of Carrier Transmissions, of which thefollowing is a specification. v j This invention relates ltotransmission Asystems in which carrier currents are employed for thetransmission of signals and more particularly to methods land meanswhereby the transmission over systems of this character may bemaintained substantially constant with changes in the condition of thecircuits,

Ina carrier transmission system the' eiiiciency of the circuit fortransmission purposes will vary from time. to time due to a Since thecarrier currents employed for transmission are of fairly high frequency,changes in the attenuation are due mainly to variations in. the leakageconditions of the open wire lines upon which the carrier channels aresuperposed. Consequently during wet weather when the leakage increasesthe circuit will be much less efficient than during dry` weather. Thisis one of the principal causes of variatlon 1n the transmissloneiliclency of carrier circuits and is quite distinct from the cause oftransmission variations in long cable c ircuits used for thetransmission of Aunder different weather conditions.

ordinary telephone frequencies Where the resistance variation withtemperature .is the controlling factor.

One of the objects of the present invention is to provide means andmethods whereby the. transmission eiiiciency of a carrier.

circuit may be maintained substantially constant under differentconditions particularly a nother object of the invention is to providemethods and means whereby the above results may be accomplishedautomatically and preferably without the adjustment of mechanicaldevices.

These objects together with other objects of the invention more fullyappearing hereinafter are. realized by taking advantage of a peculiarproperty of certain types of modulating and demodulating devices such asvacuum tubes. It has been discovered that if a vacuum tube is used as amodulator, or as a demodulatoroperating upon the homodyne principle, theeiiciency of the Seriallo. 395,111.

tube varies with the quantity or amplitude of the carrier frequencysupplied. As the amplitude of the carrier current is increased themodulated output current increases up to a certain point after which theoutput current decreases asthe amplitude of the carrier current isfurther increased. If then the carrier current which is to be suppliedto the modulating or demodulating tube is transmitted over the carriercircuitsand if under normal conditions of the circuit the amplitude ofthe carrierv current be made of such value thatthe tubeisfoperating'at apointl beyond the peak of its output eiliclency, the efficiency of thetransmission circuit will tend to vremain constant. This follows fromthefact that if, ldue to wet weather or other abnormal conditions along theline, the transmission efficiency ofthe line circuit itself is decreasedthe amplitude of the carrier current transmitted over the acorresponding increase`in the modulated output current. This increase inthe volume of the modulated output-tends to compensate for the decreasein the modulated carrier current transmitted overthe line and thus anautomatic regulation is provided which is purely electrical.

The invention may now be more fully understood from. the followingdescription when read in connection with the accompanying drawing,Figure 1 of which is a schematic circuit diagram of the terminalapparatus of a carrier transmission system and Fig.' 2 of which is acurve illustrating the variations in modulated output current of amodulating or a demodulating tube with the amplitude of the suppliedcarrier current.

Referring to Fig. 1. ML represents an ordinary transmisslon line, suchas a telephone line, said line terminatingin a terminal line LT,including the usual composite set whereby the telephone circuit may becomposited for Morse operation, and the usual phantom coil Y andvcomposite ringer Z. lSince the elements and Z are well known in the artthey arefsimply indicated schematically upon the drawing and'no furtherdescription is deemed necessary.

In order that a carrier system may be superposed upon the transmissionline LDL, a carrier branch CB is connected to the main line ML at thejunction point of said main line with. the terminal line TL, saidcarrier branch leading to terminal carrier apparatus. ln order toprevent the transmission of the carrier frequencies to the terminal lineLT and the low frequency apparatus associated therewith, a broad bandfilter LF is inserted in the terminal line LT said filter being sodesigned as to transmit frequencies 'below the upper limit of telephonietransmission and to substantially suppress frequencies above such limit.ln order to prevent the transmission of low frequency currents, such asordinary telephone or Morse currents, to the carrier branch CB, a broadband filter HF is inserted in said branch, said filter being so designedas to transmit frequencies lying above the upper limit of ordinarytelephonie transmission -while suppressing frequencies lying below saidlimit'` These filters may be of the general type -disclosed in UnitedStates patents to George A. Campbell, Nos. 1,227,113 and 1,227,114 datedMay 22, 1917. The filters, however, may be of any other type, the onlyrequirement being that they transmita band of frequencies having thelimits above referred to.

The carrierbranch CB is associated with a carrier transmitting circuitTL and a carrier receiving circuit RL to which the various carrierchannels are connected. The circuits TL and RL are associated with thecarrier branch CB in such a manner as to be conjugate with respect toeach other, that is, carrier oscillations transmitted from TL to CB willbe without effect upon RL and vice versa. This result is secured byproviding a balanced transformer arrangement 10, of well known type, anda balancing circuitN.

The carrier transmitting and receiving circuits TL and RL are associatedwith a plurality of carrier channels, which will now be described. L1,L2, L3 and L, represent low frequency telephone or telegraph lines overwhich telephonie or other signals are transmitted, which signals are to-be impressed upon carrier currents and simultaneously transmitted overthe main line ML. The line L1 is associated with a carrier transmittingcircuit TL1 and a carrier receiving circuit RL1 through a balancingtransformer arrangement 11 and a balancing network N1, the connectionbeing such that the circuits TL1 and RL1 will be conjugate with respectto each other. In a similar manner, line L2 is associated with circuitsTL2 and RL2 through transformerv 12 and network N2; line L3 isassociated with circuits TL3 and RL3 through transformer 13 and networkN3;'and line L.,L is associated with circuits TL4t and RL4 throughtransformer 14 and network N4. Each of the transmitting channels TL1,TLZ, TL8 and TL,l is connected to reeaeea the common transmittingcircuit TL while each of the receiving channels RLI, RL2, RLS and RL4 isassociated with the vcarrier receiving circuit RL.

Tn order to supply the carrier currents having dierent frequencies foreach channel, a frequency control system of-the general type disclosedin the application of B. W. 'KendalL Serial Number 130,350, filedNovember 9, 1916, is provided. The frcquency control system comprises anoscillator FG, which may be of any well known type, such as, forinstance, the well known vacuum tube oscillator, said oscillator servingto generate 'a basic carrier frequency illustrated, by way of example,in the drawing as being 5,000 cycles. 1n order to translate the basicfrequency into harmonics for the different channels, an harmonicproducer schematically indicated at HP is provided. This harmonicproducer may be a vacuum tube arrangement for distorthig the wavesgenerated by the oscillator FG- so as to produce harmonics in the outputcircuits of the distorting tube. Harmonic producers of this characterare shown and described in the above mentioned application of B. W.Kendall as well as in an application of B. W. Kendall, Serial Number139,530 filed December 29, 1916. The harmonics produced by the harmonicproducer HP, which may be for example of 10,000, 15,000, 20,000 and25,000 cycles, are filtered out and separately transmitted to theseveral carrier channels through filtering devices CF1, CFg, CE, andGF4. These filtering devices may'be broad band filters of the typedisclosed in the above mentioned applications of George A. Campbell ormay be simple tuned circuits of any well known type. The fundamentalfrequency here illustrated as 5,000 cycles may be transmitted through asimilar filtering arrangement FF to an amplifier FA. The amplifier FAmay be of any known type, such as, for instance, a vacuum tubeamplifier. The amplified fundamental frequency may be then passedthrough another filter FF and impressed upon the transmitting carriercircuit TL. The harmonics used as carrier frequencies after beingfiltered by means of the filters CF1, CF2, etc., may be impressed uponcarrier amplifiers C1, CA2, CA3 and C1114. These amplifiers may besimilar to the amplifier FA already referred to.

Each of the transmitting channels TL1, TL2, etc., is provided with amodulating arrangement schematically illustrated at M1, M2, M3 and M4.This modulating arrangement may be of any desired character but ispreferably a vacuum tube arrangement, such as is illustrated in the U.S. patent to John R. Carson Number 1,343,307, issued June 15, 1920.

with a demodulator or detector, such as 1) l illustrated and describedin the U. S.

D2, D2 and D2. These demodulating or detecting arrangements may be ofany WellV known type, but are preferably of the ttyp a en tol J. R.Carson, Number 1,343,308, lssued June 15,1920. The detecting ordemodulating arrangement of the said vpatent to J. R. Carson employs thesorcalled homodyne method of detecting, ythereby necessitating that thecarrier frequency of that particular channel be supplied to thedemodulating arrangement. Accordingly, the carrier frequency ofeachchannel after beino vamplified by the carrier amplifiers UA1, A2,etc., is supplied in parallel to the modulating arran' ement of thetransmitting channel and t e demodulating arrangement of thecorresponding'receiving channel.

The carrier oscillations imressed upon the modulator, as for instance 1,are modulated by means of the low frequency signal 'waves transmittedfrom the low frequency line, such as line L1, to the carrier channelTL1, and are then filtered and amplified. For this purpose broadbandfilters TF1, TF2, TF2 and TF2' are provided inthe output circuits of themodulators M1, M2, M3 and M1.' These filters may be of thetype disclosedin the patentsto Campbell above referred to and each filter should becapable of transmitting a band of frequencies whose Width is equal tothe range of frequencies employed in 10W frequency signaling. In thecase of telephone transmission, the filter should be so designed as totransmit a band equal to the telephonie range to the exclusion offrequencies lying Without this range. The actual frequencies transmittedby the filter of each channel will be different` for each channel'. Thefilter of the channel TL1 transmitting a band of frequencies in theneighborhood of 25,000 cycles, the filter in the next channeltransmitting a band in the vicinity of 20,000 cycles, etc. The modulatedcarrier oscillations transmitted through the filters may be amplified byamplifiers TA1, TA2, TA3 and TA, arranged in the several transmitting'channels. These amplifiers may be of any Well known type but are pref'verably vacuum tube amplifiers. -In order to further increase theselectivity, filters TF1', TF2', TF2 and TF4 may be provided in theoutput circuits of the amplifiers.

The several carrier frequencies are modulated in accordance with thelou7 frequency signals transmitted over the lines L1, L2, L1 and L4 andafter being filtered and amplified are simultaneously impressed upon thecommon carrier transmitting circuit TL, the fundamental frequency hereillustrated as being of 5,000 cycles, being at the samv time impressedupon the common circuit.

, These multiple frequencies are then transmitted to the transformervlOand over the carrier branch CB' to the mainline ML.

Owing to the balancing arrangement of the ltransformer 10 and thenetwork N, the refilters TF1 or TF1", etc., of the associatedtransmitting channels. The modulated carrier oscillations transmittedover the main line ML from a distant station pass through the filter HF,being excluded from the terminal line by filter LF, and are thenimpressed through the transformer 10 upon the' common receiving circuitRL. Modu- .lated currents corresponding in frequencyv to the differentchannels are filtered out by means of the filters RF1, BF2; etc., andimressed upon the demodulators`D1, D2, etc. lnmodulated carrierfrequencies are at the lsame time impressed upon the demodulators fromthe control circuit already described so thatthe lov7 frequency signalWaves, in accordance with which the high frequency carrier waves weremodulated, appear in the output circuits of the demodulators.

In order to amplify these low frequency Waves, low frequency amplifiersBA1, RA2,

RA3 and RA,1 are provided in the several receiving channels. Theseamplifiers may be of any Well known type but are preferablyl vacuum tubeampliers. In order to purify the lovv frequency currents from anyresidual high frequency components, low frequency filters, such as RF1',BF2', BF2 and RF1 are provided in the output circuits of the amplifiers.These filters may be of the same character as the low frequency filterLF in the terminal line LT and are of the general type disclosed in thepatents to Campbell above referred to. The amplified lorsT frequencysignal waves from the several receiving channels are then impressedthrough the transformers 11, 12, 13 and 14 upon the respective lowfrequency lines L1, L2, L2 and L2.

It will be remembered that in connection with the production of thevarious carrier frequencies, a fundamental frequency of some desiredvalue, such as 5,000 cycles, for example, Was generated and transmittedthrough the filters FF and FF and the amplifier FA to the commontransmitting circuit TL. This fundamental' frequency was thentransmitted together with various modulated carrier frequencies throughthe transformer arrangement 10 and the highv frequency filter-HF to themain line At a distant point in the main line the various carrierfrequencies may be separated from the low frequency signaling currents,

i uen() such as telephone currents, by means of a high fre uencycomposite set similar to that already escribed and comprising highfreand low frequency filters similar to e fi ters HF and LF. rlllhe.carrier frequencies may then be impressed upon terminal carrierapparatus similar to that shown in Fig. 1. The terminal carrierapparatus may dier, however, from that shown in Fig. 1 with respect tothe production of the carrier frequencies. In order to illustrate themanner in which the system will be modified at the distant station inthis respect, the apparatus of Fig. 1 has been shown with a circuit 2fleading from thev common receiving circuit RL to an harmonic producer(not shown). The filtering arrangement, such as a tuned circuit FR,is'included in the circuit 21 to filter out the fundamental frequencyvof 5,000 cycles transmitted over the line. This frequency is thenY:impressed by means of the circuit 21 upo an harmonic reproducer whichwill be in general similar to the harl monic producer HP and the generalprinciple of this arrangement is described in the application of B. W.Kendall, Serial Number 139,530, filed December 29, 1916, alreadyreferred to. The general Aarrangement whereby the harmonic reproducer isassociated with the receiving circuit is shown and described in theapplication of B. W.

Kendall, Serial Number 130,350, led .No-v

vember 9, 1916, to which reference has already been made. it issufficient for the purposes of this application to state' that theharmonic reproducer functions to produce harmonics of the basicfrequency in a manner similar to the harmonic producer HP alreadydescribed, the several harmonics being filtered out, amplified to adesired is derived from this fundamental frequency.

Consequently, the amplitude of the carrier frequencies supplied to thesetubes at the distant terminal station will vary as the amplitude of thefundamental frequency varies, which fundamental frequency in turn varieswith changes in the transmission efficiency of the main line due todierent weather conditions and the like.

messes As previously stated the output of modulated current from thevmodulating or demodulating tubes varies in a peculiar manner withincrease in the amplitude of the carrier current supplied to .the tube.rlhis variation is illust-rated by the curve 20 in Fig. 2. It isapparent from an inspection of thecurve that as the amplitude of thecarrier-current is increased the modulated output current also increasesuntil the point 21 is reached after which the output of modulatedcurrer." decreases with an increase in amplitude ot current. If thenwith the line operating under normal conditions the amplitude of thefundamental frequency supplied at the generating station be made suchthat the carrier currents derived therefrom atthe vdistant station willbe of the amplitude indicated at 22. the modulated output current of themodulators and demodulators at the distant station will be less than themaximum. Should the transmission efficienc of the main line decrease dueto wet weat er, for instance, the amplitude of the fundamental frequencyarrivlng at the distant station will decrease, with a correspondingdecrease in the amplitudes in the carrier currents supplied to thevarious modulating and demodulating tubes. For a given modulatingcurrent, whether high frequency or low frequency, supplied to the tube,the modulated output current would be decreased.

Let us consider the effect of this as applied to transmission from thestation at which the fundamental frequency is generated to the distantstation. The modulated current supplied at the generating station to theline as' a result of the modulation of a carrier current by a signalwill have the same amplitude when applied to the line regardless ofconditions on the main line since the fundamental frequency genera-tedat that station together with the carrier currents derived therefromwill be the same. lf the attenuation of the main line is greater thannormal, however, the modulated carrier currents will be decreased inamplitude to a greater degree than normal upon arriving at the distantstation. rllhe fundamental frequency transmitted over the main linewill, however, arrive at the distant station correspondingly diminishedthereby tending to increase the output of the modulating anddemodulating tubes. The output of the demodulating tubes, however, willnot actually be increased because the modulated carrier currentssupplied to the tube after transmission over the line have beendecreased by corresponding amounts. Consequently, the actual lowfrequency output of the demodulating tube will be substantially the sameas under normal conditions.

In the case of transmission from the distant station to the terminalstation at which the fundamental frequency is generated, if

the attenuation of theqmain line is greater` than normal the amplitudeof the fundamental frequency will be decreased below its normal valueupon arriving at the distant station. This results in a carrier currentof increased amplitude being supplied to each of the modulating tubesand assuming that the low frequency signals applied to the modulators-are of normal amplitude the modulated high frequency output currentssupplied to the line Will be of increased amplitude as compared withnormal. This increase in the amplitude of the modulated carrier currentssupplied to the line from the distant station Will be about sullicientto make up for the increase in the attenuation of the line so that thecurrents will arrive at the enerating station with their amplitudes aout normal.v Since the amplitudes of the carrier frequencies supplied tothe demodulating tubes at the generating station do not vary Withchanges in the condition of the main line the low frequency demodulatedoutput currents supplied to the terminal loW frequency lines Will havesubstantially the same values as under normal conditions.

It will beobvious that the general principles herein disclosed may beembodied inI many' other organizations widely different from thoseillustrated Without departing from the spirit of the invention asdefined inthe following claims.

What is claimed is:

1. The method-of maintaining the transmission of a carrier circuitsubstantially constant for all carrier frequencies transmitted thereoverregardless of changes in theline condition, which consists in subjectinga pilot carrier current to the same transmission conditions as those towhich the various carrier currents employed for the transmission ofsignals are subjected,

and controlling the amplitudes of the car.

rier currents employed for signaling transmission 1n accordance Withvariatlons 1n the amplitude of the pilot carrier current inv tion, whichconsists in generating at one terminal station a fundamental frequencyfrom which all other carrier frequencies for use at said station arederived, transmittmg said'ifundamental frequency over vthe mainline tothe distant station, deriving from said fundamental frequency at saiddistant station carrier frequencies for use at said station, andmaintaining the amplitude of the generated fundamental at such valuesthat under normal line transmission conditions the amplitudes of thelderived carrierkfrequencies at the distant station Will be greater thanthe amplitude necessary for maximum output of the modulating anddemodulating devices.

3. In a carrier system including a line circuit and modulating anddemodulating devices operating most eiiiciently at a certain inputamplitude and less efciently for greater or lesser amplitudes, themethod of maintaining the transmission of a carrier circuitsubstantially constant for all carrier frequencies transmitted thereoverregardless of changes in the line transmission condition, which consistsin generating at one terminal station a fundamental frequency from whichall other carrier frequencies for use at said station are derived,transmitting said fundamental frequency over the main line to thedistant station, deriving from said fundamental frequency at saiddistant station carrier frequencies for use at said Station, andmaintaining the amplitude of the generated fundamental at such valuesunder normal line transmission conditions that the amplitudes ofthecarrier frequencies supplied to the demodulating devices at thedistant station Will be greater than the amplitudes necessary formaximum output of said demodulating devices by an amount such that theefficiency of the demodulating devices Will increase as the attenuationof the line increases to offset the increased attenuation of themodulated carrier currents transmitted over the line to the distantstation.

j 4. In a carrier system including a line circuit and modulating anddemodulating devices operating most efficiently at a certain inputamplitude' and less efficiently for greater or lesser amplitudes, themethod of maintaining the transmission of a carrier circuitsubstantially constant for all carrier frequencies transmitted thereoverregardless of changes in the line transmission con-4 dition, which'consists in' generating at one terminal station a fundamental frequencyfrom which all other carrier frequencies for use at said station arederived, transmitting said fundamental frequency over the main line tothe distant station, deriving from said fundamentalfrequency at saiddistant station carrier frequencies for use at said station, andmaintaining the amplitude of the generatedl fundamental under normalline transmission conditions at such values that thefamplitudes of thecarrier frequencies derived therefrom at the distant sta- @3 LacasseYmodulating devices, whereby as the attenuation of the line is increasedthe eiiciency of the modulating devices will be correspondinglyincreased so that the output currents in the modulators when transmittedover' the line Will arrive at the station at which the fundamentalfrequencies were generated with substantially the same amplitude asunder normal conditions.

5. The method of maintaining the transmission of a carrier circuitsubstantially constant regardless of changes in the line trans- -missioncondition, Which consists in transmitting the carrier current over saidcircuit and in translatingthe transmitted carrier current into anothercurrent so related to the transmitted current that a decrease in theamplitude of the translated carrier current will produce an increase inthe amplitude of the signaling current detected from the translatedcurrent.

6. A carrier Wave transmission system in which signal variations ofcarrier Waves are produced and transmitted to a distant station over asuitable transmitting medium subject to changes in transmissioneiiiciency, modulators associated With said transmitting medium at theterminal station, said modulators being so arranged that for normaltransmission conditions of the transmitting medium the unmodulatedcarrier component supply exceeds the value for which said modulatorsoperate `at maximum efiiciency, whereby poor transmission conditions ofthe transmitting medium and a consequent decrease in applied unmodulatedcarrier comonent are accompanied by an increased efciency of theover-all transmission of the circuit including the transmitting mediumand modulators.

7. A carrier Wave transmission system in which signal variations ofcarrier waves are produced and transmitted to a distant station over asuitable transmitting medium subject to changes in transmissionefliciency, demodulators for said Waves associated with saidtransmitting medium at the receiving station, said demodulators being soarranged that for normal transmission conditions of the'transmittingmedium the unmodulated carrier component supply exceeds the value 'forwhich said demodulators operate at maximum eliciency for reproducingsignals, whereby poor transmission conditions of the transmitting medium'and a consequent decrease in applied unmodulated carrier component areaccompanied by an increased efficiency in the reproduction of thesignals.

8. In a carrier Wave transmission system inwhich signal variations ofcarrier Waves are produced and transmitted to a distant station over asuitable transmitting'medium subJect to changes in transmission eclency,

modulators and demodulators for said Waves,

said modulatorsv and demodulators being so arranged that 'for normaltransmission conditions of the transmitting medium the unmodulatedcarrier component supply exceeds the value :tor Which said modulatorsand demodulators operate at maximum eliciency for reproducing signals,whereby poor transmission conditions of the transmitting medium and aconsequent decrease in applied unmodulated carrier component areaccompanied by an increased eliciency in the reproduction of signals.

9. A carrier Wave transmission system in which signal variations ofcarrier Waves are produced and transmitted to a distant stav tion, apair of translating arrangements corresponding to each carrier Wave, oneof said translating arrangements comprising a modulator and the othercomprising a detector, one of said translating arrangements being soarranged-that for normal transmission conditions of the `transmittingmedium the unmodulated carrier component supply exceeds the value forwhich said translating arrangement operates at maxi mum eiiclency,whereby poor transmission conditions of the transmitting medium and theconsequent decrease in applied unmodulated carrier component areaccompanied by an increased efficiency of the over-all transmission ofthe circuit, including the transmitting medium and translatingarrangements.

ln testimony whereof, I have signed my name to this specification this30th day of June 1920. 'f

HAROLD S. OSBORNE

